In an electric powered steering apparatus of an automobile, for example, a steering torque sensor senses a steering torque applied to a steering shaft by the driver using a steering wheel. A magnetostrictive torque sensor has recently been proposed as such a steering torque sensor. The steering shaft bears rotational force produced by steering operations, and functions as a rotating shaft. The steering shaft functions as part of the rotating shaft in the mounting location of the steering torque sensor. The electrically powered steering apparatus controls the driving of a steering force auxiliary motor according to a torque signal sensed by the steering torque sensor. The electrically powered steering apparatus reduces the steering force for the driver to provide a pleasant steering feel.
In the steering torque sensor of the electrically powered steering apparatus, magnetostrictive films that are magnetically anisotropic with respect to each other are formed at two specific locations on the surface of the steering shaft. The magnetostrictive torque sensor has a configuration in which a non-contact manner is used to detect changes in the magnetostrictive characteristics of the magnetostrictive films that correspond to the torsion of the steering shaft when torque is applied to the steering shaft from the steering wheel.
The method for manufacturing a magnetostrictive torque sensor comprises forming a magnetostrictive film over a specific surface in part of the steering shaft; i.e., over a circumferential surface having a specific axial width in the rotating shaft, and providing the magnetostrictive film with magnetic anisotropy. Conventional methods for providing the magnetostrictive film with magnetic anisotropy in the manufacture of a magnetostrictive torque sensor involve forming a magnetostrictive plating (magnetostrictive film) on a rotating shaft by electro-plating, for example, applying a twisting torque to the rotating shaft to create stress in the circumferential surface of the rotating shaft, and heat-treating the rotating shaft in a thermostat while the shaft is kept under stress (see JP-A-2002-82000, for example).
A magnetostrictive torque sensor for use as a steering torque sensor of an electrically powered steering apparatus is preferably one that is highly functional and has low nonuniformity in the sensor characteristics. In a magnetostrictive torque sensor, a structural steel that is inexpensive and has excellent strength and machining characteristics is used as the steering shaft on which a magnetostrictive film is formed.
SK, SCM, SNC, or other materials are used as the structural steel. These materials are magnetic materials, and include non-magnetic residual austenite as a component. In a steering shaft composed of structural steel, the residual austenite content is different depending on the heat-treatment conditions (hardening, tempering, and other conditions), composition, and other factors in the manufacturing stage. When the residual austenite content is different, the magnetic characteristics of the steering shaft change. In a method of manufacturing a common steering shaft, a steering shaft is ordinarily delivered containing residual austenite that has a content nonuniformity in a range of about 5 to 15%. Heat treatment is thereafter carried out in order to manufacture a magnetostrictive film on the steering shaft. It is known that, depending on the heat treatment, the heating state of the magnetostrictive film varies in accordance with the difference in the magnetic characteristics of the steering shaft, and the ultimate sensing characteristics of the sensor of the magnetostrictive torque sensor vary considerably. When the residual austenite content varies by 10%, for example, the ultimate sensing characteristics of the sensor vary by 10% or more.
As described above, the present inventors discovered that due to the difference in the residual austenite content of the steering shaft, temperature differences occur in the vicinity of the surface of the steering shaft even in cases in which the same heating conditions are maintained in the heating treatment performed when magnetic anisotropy is provided to the magnetostrictive film formed on the surface of the steering shaft. For this reason, nonuniformity in the sensitivity characteristics of the manufactured magnetostrictive torque sensor is produced when there is a difference in the residual austenite content.
Reducing the nonuniformity of residual austenite content in the steering shaft as such can be considered as a method of reducing nonuniformity in the sensitivity characteristics of a magnetostrictive torque sensor. However, a high level of technical difficulty is required to regulate the residual austenite content of the steering shaft on the order of several percent in the manufacturing stage. The cost of the steering shaft therefore becomes very high even if a steering shaft in which the residual austenite content is regulated were to be manufactured.
On the other hand, when a steering shaft in which the residual austenite content is different from the commonly used content, and a magnetostrictive torque sensor is manufactured in accordance with a conventional manufacturing method, the sensitivity characteristics of the magnetostrictive torque sensor thus manufactured become more nonuniform. For this reason, workers were required to adjust the sensitivity characteristics of the magnetostrictive torque sensor when the magnetostrictive torque sensor was mounted in the electrically powered steering apparatus. As a result, the process for assemblying the electrically powered steering apparatus was made more complicated, and the production costs of the apparatus were also increased.
When the sensitivity characteristics of the completed magnetostrictive torque sensor vary 10% or more, sensor production yield is reduced and adjustment of the characteristics in the assembly process becomes very difficult.
There is therefore a need for a method of manufacturing a magnetostrictive torque sensor that can reduce nonuniformity in the sensitivity characteristics of the sensor by ascertaining the residual austenite content, which is in a non-magnetic phase in a rotating shaft made of a magnetic material, and regulating the heat-treatment time and other parameters of the magnetic film. There is furthermore a need for a method of manufacturing a magnetostrictive torque sensor whereby the sensor production yield can be improved, the magnetostrictive torque sensor can be easily mounted at low cost, the assembly steps can be simplified overall, and the burden on workers can be improved.